Allochem is a term introduced by Folk to describe the recognisable 'grains' in carbonate rocks. Any fragment from around .5 mm upwards in size may be considered an allochem. Examples would include ooids, peloids, oncolites, pellets, fossil or pre-existing carbonate fragments. Fragments are still termed allochems if they have undergone chemical transformations - for example if an aragonite shell were to dissolve and be later replaced by calcite, the replacement would still be deemed an allochem. [Source: <a href="https://en.wikipedia.org/wiki/Allochem" target="_blank">en.wikipedia.org/</a> ]

Amphibole is the name of an important group of generally dark-colored, inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO 4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles can be green, black, colorless, white, yellow, blue, or brown. [Source: en.wikipedia.org/ ]

Perthitic texture are parallel lamellae, or trains of blebs, of one mineral with the same optical orientation that are enclosed in a single 'host' crystal of another mineral. Antiperthitic texture reverses the typical pattern of lamellae or blebs of soidum-rich feldspar in a host of potassium-rich feldspar.
[Source: MacKenzie et al. 1982]

Aphantic is a textural term applied to igneous rocks used to describe fine-grained rocks in which all crystals, other than phenocrysts, cannot be seen with the naked eye. Most fine-grained volcanic rocks are aphantic. This term is in common use. [Source:wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Aplitic rocks is the name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens. Dykes and threads of aplite are commonly observed traversing granitic bodies; they occur also, though less frequently, in syenites, diorites, quartz-diabases and gabbros. [Source: en.wikipedia.org/ ]

Augen are eye-shaped mineral grains or aggregates of mineral grains found within sheared, foliated metamorphic rocks such as gneisses. Augen form by growth of a mantle on a porphyroblast or porphyroclast with preferential growth occurring in low stress regions in the pressure shadow. Rotation of augen during growth, due to shearing, leads to sigmoidal inclusion trails and augen tails at different levels in the external foliation. Drag folding may also be observed in augen. Gneiss containing abundant augen are sometimes termed augen gneiss. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

An authigenic mineral or sedimentary rock deposit is one that was generated where it is found or observed. Authigenic sedimentary minerals form during sedimentation by precipitation or recrystallization instead of being transported from elsewhere (allogenic) by water or wind. [Source: en.wikipedia.org/ ]

Bioclasts are skeletal fragments of marine or land organisms that are found in sedimentary rocks laid down in a marine environment—especially limestone varieties, some of which take on distinct textures and coloration from their predominate bioclasts—that geologists, archaeologists and paleontologists use to date a rock strata to a particular geological era. [Source: en.wikipedia.org/ ]

Chemical zoning is a feature of a crystal where different growth zones of the crystal have different compositions. Chemical zones trace the outline of the crystal during its growth. Chemical zoning occurs only in minerals that belong to solid solution series that crystallise with different compositions at different temperatures (and/or pressures). It also indicates disequilibrium since for chemical zones to be preserved chemical diffusion must be sufficiently impeded to prevent homogenisation of the composition of a crystal. Usually chemical zoning indicates higher cooling rates. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Chert is a rock consisting of microcrystalline, cryptocrystalline or microfibrous quartz crystals. Chert is often found as nodules formed during diagenesis of carbonate-bearing sediments and is a common clast within siliciclastic rocks such as conglomerates. Some chert is deposited due to the accumulation of siliceous foraminifera which dominates deep-water oozes below the carbonate compensation depth. Chert beds dominated by radiolarians are termed radiolarites. Diatomaceous cherts are also known and form during diagenesis of diatomaceous oozes. Some chert can form by precipitation of silica from hydrothermal fluids, but these are more properly termed geyserites or silica sinters. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn. Chlorite is commonly found in igneous rocks as an alteration product of mafic minerals such as pyroxene, amphibole, and biotite. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a metasomatism product via addition of Fe, Mg, or other compounds into the rock mass. Chlorite is a common mineral associated with hydrothermal ore deposits and commonly occurs with epidote, sericite, adularia and sulfide minerals. Chlorite is also a common metamorphic mineral, usually indicative of low-grade metamorphism. It is the diagnostic species of the zeolite facies and of lower greenschist facies. It occurs in the quartz, albite, sericite, chlorite, garnet assemblage of pelitic schist. Within ultramafic rocks, metamorphism can also produce predominantly clinochlore chlorite in association with talc. [Source: en.wikipedia.org/ ]

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits. [Source: en.wikipedia.org/ ]

A clay is a soft, cohesive, water-rich mudstone that exhibits a plasticity and will harden when fired. The defining feature of clay, plasticity, originated due to surface interactions of clay grade grains in the presence of water and depends on the surface properties of these grains. The majority of clays are dominated by phyllosilicates, including the clay minerals. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ] Per the ISO system, clay particles are ≤.002 mm.

Comb layering is a texture in an igneous rock where dendritic crystals, often curved and branching, are oriented subparallel to each other. Dendrites are often crystals of pyroxene and/or olivine. Comb layering occurs during crystallisation at large supercooling where crystals nucleate on a planar substrate, such as the walls of a dyke. They are most commonly found in chilled margins. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A conglomerate is a sedimentary clastic rock consisting of rounded fragments of a pre-existing rocks, larger than 4 mm, set within a finer-grained sedimentary matrix. Rocks with angular clasts are known as breccias, those with angular and rounded clasts are sometimes termed breccia conglomerates. Conglomerates are characteristic of the deposition of transported materials in high energy environments. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Coquina is a sedimentary rock that is composed either wholly or almost entirely of the transported, abraded, and mechanically-sorted fragments of the shells of either molluscs, trilobites, brachiopods, or other invertebrates. For a sediment to be considered to be a coquina, the particles composing it should average 2 mm or greater in size. Coquina can vary in hardness from poorly to moderately cemented. [Source: en.wikipedia.org/]

A corona texture is a texture in which a host crystal is a mantled by one or more crystals of another mineral. Often corona textures arise due to reaction between the host crystal and the surrounding material to generate the mantling phase, however, some corona textures also arise by epitaxial growth involving the nucleation of the mantling phases on the host crystal without reaction. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A crystal clot is an aggregate of intergrown crystals within an igneous rock in which component crystals are larger than the surrounding matrix. Crystal clots are closely related to glomerocrysts that represent aggregates of largely separate phenocrysts. They are common in fine-grained igneous rocks. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Detrital refers to particles of rock derived from pre-existing rock through processes of weathering and erosion. A fragment of detritus is called a clast. Detrital Particles can consist of lithic fragments (particles of recognisable rock), or of monomineralic fragments (mineral grains). [Source: en.wikipedia.org/ ]

Diorite is a coarse-grained intermediate igneous rock consisting of essential andesine plagioclase and one or more mafic minerals, usually hornblende, biotite or pyroxene. Hornblende is the most common mafic found in diorite. Pyroxene can include both orthopyroxene and clinopyroxene. Minor amounts of quartz, alkali feldspar, feldspathoid and olivine can occur in diorites. Those containing >5% quartz are known as quartz diorites. Those containing more than 10% feldspathoid are known as monzodiorites. Technically diorite is distingushed from gabbro by the composition of its plagioclase and both gabbro and diorite plot in the same field on a QAPF diagram. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A dunite is an ultrabasic igneous rock dominated by essential olivine (>90% volume), often with accessory clinopyroxene, orthopyroxene, spinel, ilmenite, and magnetite. Dunite is usually coarse- to medium grained and is a peridotite. Dunite forms either as a cumulate within layered intrusions or as a residue after extraction of partial melt from a pre-existing ultrabasic rock in the mantle. Dunite cumulates and mantle rocks are found as xenoliths in a wide range of mantle derived magmas. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

An embayed crystal in an igneous rock is a crystal with an irregular cavity penetrating a crystal face. The embayment is often filled with groundmass or another mineral. Embayments in crystal faces may indicate the crystal is out of equilibrium and reacting with the surrounding melt, especially when combined with rounded crystal shapes and reaction rims. Embayments, however, can arise through imperfections in crystal growth and impinging crystals. Voids and cavities in skeletal crystals are also often described as embayments, however, these arise due to crystal growth under large supercooling. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Euhedral is a textural term used to describe crystals that are completely bound by characterisitic faces. The terms idiomorphic and automorphic are synonymous with the term euhedral but are not commonly used. Idioblastic is a term applied to euhedral minerals within metamorphic rocks.[Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A eutaxitic texture is a fabric observed within ignimbrites in which discontinuous layering is formed by flattening of pumice fragments. Eutaxitic textures form within welded ignimbrites that were emplaced hot and collapsed under their own weight. Flattened pumice fragments are termed fiamme and when fresh are highly vesicular glassy objects and often dark and vitreous in hand specimen. Devitrification and alteration, however, can produce dull pale coloured fiamme. Fiamme vary considerably from lapilli to block-sized and are usually contained within a groundmass consisting of flattened glass shards and crystal fragments. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

F

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust. Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite.[4] Feldspars are also found in many types of sedimentary rocks. [Source: en.wikipedia.org/ ]

A foliation is a general term for a planar fabric in a rock. It is most commonly applied to planar tectonic fabrics in metamorphic rocks such as schistosity and slaty cleavage. A tectonic foliation often arises due to the alignment of elongate or platy minerals due to deviatoric stress and are perpendicular to the principle stress direction. They may also form a grain flattening fabric by elongation of equant grains perpendicular to the principle stress direction. At high metamorphic grades mineral segregation can occur to produce compositional layering, or gneissose banding. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

G

A gabbro is the coarse-grained equivalent to a basalt - a basic igneous rock dominated by calcic plagioclase, pyroxene and/or hornblende. Pyroxene in gabbro can be clinopyroxene and/or orthopyroxene, usually pyroxenes with intermediate calcium contents exhibit exsolution. Pigeonite, when present, has inverted to orthopyroxene. Gabbro dominated by orthopyroxene is known as a norite. A pyroxene-poor gabbro is a troctolite. Plagioclase is more calcic than andesine (50% anorthite). Gabbro dominated by anorthite has been termed eucrite. Gabbro often also contains olivine and/or quartz and can be silica undersaturated, saturated or oversaturated. The term tholeiitic can be applied to silica saturated or oversaturated gabbros. Feldspathoids can be found in silica-undersaturated alkali gabbros. Gabbro can contain abundant hornblende and can include biotite. In a QAPF diagram gabbro occupies the same field as diorite. The two are distinguished by the calcic nature of plagioclase in gabbro. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A gneiss is a coarse-grained, foliated, metamorphic rock typically containing amphibole, muscovite, biotite, quartz and feldspar. Feldspar, which is usually oligoclase or microcline, also commonly occurs. A wide range of minerals are important as accessories and are characteristic either of the protolith composition or metamorphic grade. These minerals include graphite, garnet, staurolite, kyanite, sillimanite, amphiboles, and epidote. Porphyroblasts, crystals larger than average grain-size, can occur in gneisses. Compositional banding into mafic-rich, amphibole and mica-rich, and quartz-feldspathic bands is a definiting feature of gneiss, they usually lack a well defined schistose parting. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Granite is a medium- to coarse-grained acid igneous rock with essential quartz (>20%) and feldspar, where alkali feldspar constitutes between 100 and 35% of the feldspars, and minor mafic minerals. Hornblende and biotite are the commonest mafic minerals, however, muscovite is also frequently encountered. Pyroxene, andalusite, corundum, tourmaline, garnet and topaz are also relatively common mafic minerals. Rhyolite is the fine-grained equivalent of granite. The term granitic is often used to describe rocks of the same composition as granites. Rhyolite is, therefore, a granitic rock. Medium-grained varieties are often termed microgranites. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A granoblastic texture is an equigranular texture in which crystals adopt a polygonal morphology with grain triple junctions of approximately 120 degrees. The formation of granoblastic textures occurs to minimise the combined surface energy of phases within a rock. Granoblastic textures are often seen in the products of thermal metamorphism such as hornfels, and in high grade granulites. Rocks dominated by platy, tabular or prismatic crystals tend not to form granoblastic textures and develop decussate textures to minimise surface energy. Groundmass minerals in some igneous can develop similar polygonal textures due to limited space during crystal growth. These are, however, not termed granoblastic. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A graphic texture is an igneous rock texture in which an intergrowth of two minerals has the appearance of runic writing. Rocks containing abundant graphic textures can be described as granophyric. Graphic textures are most commonly intergrowths of quartz and alkali feldspar in which the quartz appears as v-shaped inclusions enclosed by the feldspar. A micrographic texture is a graphic intergrowth than can only be observed under the microscope. Graphic textures have been suggested to originate by simultaneous crystallisation of phases from a silicate melt in the presence of a hydrous fluid phase. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Holohyaline is a textural term applied to igneous rocks that are entirely composed of glass. The terms hyaline, vitreous and glassy are broadly synonymous with holohyaline except they imply a finite crystalline component. Glassy rocks are usually extrusive and have either been rapidly cooled, had high initial melt temperatures, or were very viscous. Obsidian is a holohyaline rock. It also includes rocks that were glassy but have now devitrifed such as the pitchstones. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A hornfels is a fine-grained, granular metamorphic rock with interlocking randomly oriented crystals and is formed by thermal metamorphism. Hornfels generally lack foliation and are hard rocks that have no preferrential plane of fracture. Their textures evolve towards granoblastic or decussate and form in order to minimise overall surface energy. Lower grade hornfels often include porphyroblasts and inclusion-rich phases. Mineralogy of hornfels is highly variable and varies with the nature of the protolith, pressure and temperature, and composition of co-magmatic fluids that may be present. Important minerals include albite, chlorite, biotite, epidote, hornblende, pyroxene, cordierite, andalusite, sillimanite, kyanite and sanidine. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Hypohyaline is a textural term applied to igneous rocks that described a rock consisting of crystals in a glassy groundmass where the ratio of crystals to glass is in the range 3:5 to 1:7. The terms hyaline, vitreous and glassy are broadly synonymous with hypohyaline but not a rigorously defined. A hypohyaline rock can be described as a vitrophyre where crystals are phenocrysts. Usually devitrified glass it treated as glass in defining a rock as hypohyaline. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

I

An ignimbrite is a welded pyroclastic rock that contains abundant flattened juvenile clasts often originally pumice. The flattened clasts within ignimbrites are termed fiamme and range from lapilli-sized (>2 mm) to block-sized (>64 mm). The layered texture produced by fiamme is termed a eutaxitic texture. The groundmass of ignimbrites is usually dominated flattened vitric shards, but can contain lithic and crystal fragments. The fine-grained groundmass of many ignimbrites has a reddish colour due to high temperature oxidation of iron, in particular in the upper parts of a pyroclastic flow deposit. Less welded flows tend to be white or grey, whilst intensely welded flows are often dark grey to black. Recrystallisation and alteration of glass within ignimbrite is common, in particular in ancient examples. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

1. In reference to voids or inclusions occurring along the boundaries between the crystals or grains of a material. 2. Texture: Intergranular is a textural term indicating that a crystal occupies the angular space between at least two larger crystals. Intergranular crystals have crystallised later than the crystals that control their shape. The term intergranular can be used for either igneous or metamorphic rocks. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Intersertal is a textural term used to denote that the angular spaces between larger crystals is occupied by glass, or glass and small crystals. The glass may be devitrified or altered to other phases.
[Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/]

K

L

A laterite is an aluminium and/or iron-rich soil formed by significant in-situ weathering in tropical areas with significant rainfall. Long-term chemical weathering of pre-existing rocks under wet and humid conditions, known as lateritization produces laterites. These soils are often a rust-red colour due to the present of hematite and goethite. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Limestone is a sedimentary rock composed of carbonate minerals, mainly calcite and aragonite, polymorphs of CaCO3. Limestones comprise around 10% of sedimentary rocks and most are generated by the accumulation of fragments of skeletal materials of marine organisms such as molluscs, corals and foraminifera. Limestones can, however, also form by secretion of carbonate my micro-organisms and inorganic chemical precipitation. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

https://en.wikipedia.org/wiki/MicaThe mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms. [Source: en.wikipedia.org/ ]

Micrographic texture is a fine-grained intergrowth of quartz and alkali feldspar, interpreted as the last product of crystallization in some igneous rocks which contain high or moderately high percentages of silica.
[Source: en.wikipedia.org/ ]

A migmatite is a banded, granular metamorphic rock that contains light coloured bands with evidence for partial melting. Migmatites comprise leucosome bands, comprised largely of quartz and feldspar, which crystallised from partial melts or segregated metamorphic fluids, and mesosomes and melanosomes that represent the residues of partial melting and are often dominated by mafic components such as amphibole, pyroxene and biotite and are often finer-grained than leucosomes. The terms neosome and palaeosome are used to denote components of the migmatite that have experience significant and little partial melting respectively. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Monzonite is a coarse-grained igneous rock with less than 20 vol% quartz and roughly equal proportions of alkali and plagioclasefeldspar. Mafic minerals present in monzonite include clinopyroxene, orthopyroxene, olivine, biotite and hornblende. Monzonite with 10-20% quartz is termed a quartz monzonite. Monzonites can also contain feldspathoids, but such rocks lack quartz. Olivine-bearing monzonites have been termed kentallenite. Hypersthene-bearing monzonites have been termed mangerite. The fine-grained equivalent of monzonite are latite and trachybasalt. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A mylonite is a metamorphic rock formed by ductile deformation of a protolith with a large shear strain. The characteristic feature of mylonites is grain-size reduction through crystal-plastic deformation that results in a rock with a strong foliation produced by ribbon structures. Larger crystals within mylonites are porphyroclasts that are relict grains that have experienced less grain-size reduction than the surrounding matrix. These are typically feldspars or garnet. Porphyroclasts in mylonites often form eye-shaped augen due to the development of pressure shadows during rotation of the crystal. Mineral grains within mylonites include intensely strained flattened grains, usually with undulose extinction, and unstrained secondary crystals formed by syntectonic or post-tectonic recrystallisation. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Neomorphism refers to the wet metamorphic process in which diagenetic alterations systematically transform minerals into either polymorphs or crystalline structures that are structurally identical to the rock(s) from which they developed. [Source: en.wikipedia.org/ ]

Normal zoning is a form of chemical zoning in which a crystals composition varies continuously from a core representing a high temperature composition of the mineral to a rim with a lower temperature composition of the mineral. Zoning forms in minerals that form a solid solution series in which mineral composition changes with temperature (and pressure). Crystals exhibiting normal zoning are said to be normally zoned. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

O

Ocellar texture is a texture found in igneous rocks in which platy or prismatic minerals are arranged radially around a central different mineral grain. The resulting sub-spherical objects are known as ocelli. The term ocelli has sometimes also been used to describe any rounded object in an igneous rock, such as immiscible blebs and mariolitic cavities. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Ooids are small (≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. [Source: en.wikipedia.org ]

Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites. The term oolith can refer to oolite or individual ooids. [Source: en.wikipedia.org/ ]

Oscillatory zoning is a form of chemical zoning in which the chemical zones oscillate in a regular fashion through a crystal. Chemical zones repeat at regular distance in oscillatory zoning, although there may be an overall normal or reverse sense to the change in composition. Chemical zoning exhibiting irregularly spaced repetitions is sometimes distinguished with the term multiple zoning. Oscillatory zoning is usually occurs due to growth of a crystal within convection currents in a magma chamber such that circulation causes quasi-regular changes in temperature and pressure. Oscillatory zoning is particularly common in plagioclase phenocrysts within dacites and andesites. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

1) An overgrowth rim is a relict imprint of a detrital margin that has been overgrown by a cementing matrix of the same mineral and maintained crysatllographic continuity through syntaxial overgrowth.

2) An overgrowth texture is a texture where a crystal has a rim of another mineral or minerals. The rim can be of the same mineral as the host crystal, or of different minerals. There are three varieties of overgrowth texture: (a) dendritic or skeletal overgrowths, (b) corona textures and (c) chemical zoning. Growth zones within otherwise homogeneous crystals could also be considered overgrowth textures and are sometimes visible by trails of inclusions. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

P

Dunham (1962) classification: a limestone of >10% allochems that is grain supported and contains micrite (carbonate mud). A packstone is a carbonate rock that is grain-supported and have a matrix of micrite (lime mud). Grains (allochems) within a packstone are largely in contact with each other. The term packstone is part of the Dunham Classification of carbonate rocks. The term is often prefixed by the most abundant allochem (grain) type, for example, oolitic packstone. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

The pericline law has [010] as the twin axis. As stated above, pericline twinning occurs as the result of monoclinic orthoclase or sanidine transforming to microcline (all have the same chemical formula - KAlSi3O8). Pericline twinning usually occurs in combination with albite twinning in microcline, but is only observable with the polarizing microscope. The combination of pericline and albite twinning produce a cross-hatched pattern, called tartan twinning, as discussed above, that easily distinguishes microcline from the other feldspars under the microscope. [Source: www.tulane.edu/~sanelson/eens211/twinning.htm ]

Perthitic texture are parallel lamellae, or trains of blebs, of one mineral with the same optical orientation that are enclosed in a single 'host' crystal of another mineral. Most common are lamellae or blebs of soidum-rich feldspar in a host of potassium-rich feldspar.
[Source: MacKenzie et al., (1982)]

Phaneritic is a term usually used to refer to igneous rock grain size. It means that the size of matrix grains in the rock is large enough to be distinguished with the unaided eye as opposed to aphanitic grains (which are too small to be seen with the naked eye).
[Source: en.wikipedia.org/]

A phonolite is a fine-grained alkaline igneous rock dominated by essential alkali feldspar, usually sanidine or anorthoclase, with alkali pyroxenes, usually aegirine-augite or aegirine, and/or alkali amphiboles, and feldspathoid. Titanite is a common accessory mineral, olivine occurs but is rare. Tephritic phonolite may contain plagioclase feldspars. Alkali feldspar, and feldspathoids, such as nepheline or leucite, are the common as phenocrysts found in phonolites. They often also contain sodalite group minerals. Some phonolites exhibit tracytic textures similar to the trachytes. Phonolites are the fine-grained equivalents of feldspathoid syenite. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A phyllite is a fine-grained, foliated, pelitic metamorphic rock with a pervasive cleavage produced by alignment of micas. Pelites are metamorphosed argillaceous sediments, such as shale and mudstone, and are dominated by muscovite, chlorite and biotite, often with graphite and iron oxides. They may also contain albite, quartz and zircon. Phyllite may contain laminations in which quartz and feldspar are more abundant and represent original sedimentary layering, they may also be interbedded with quartzite. Phyllites are typically grey to black in colour with a silky lustre on their pervasive cleavage planes due to the alignment of mica minerals. Often phyllites exhibit a crenulation cleavage which can be seen as a crenulation on the cleavage surface. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A pitchstone is a termed used to describe very fine-grained dull rocks formed by crystallisation of a volcanic glass such as obsidian. Pitchstones are usually lavas and often have rhyolitic or dacitic compositions. Crystallisation of pitchstones occurred largely in the solid state by devitrification of the glass. Pitchstones can have phenocrysts formed prior to solidification, however, their groundmass contains crystallites often with dendritic or fibrous habits. Subsequent recrystallisation can lead to granular textures that superficially resemble chert and agate. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A poikilitic texture is a texture in which small, randomly orientated, crystals are enclosed within larger crystals of another mineral. The term is most commonly applied to igneous rock textures. The smaller enclosed crystals are known as chadacrysts, whilst the larger crystals are known as oikocrysts. The most common poikilitic texture involves plagioclaselaths enclosed by augite and is known as an ophitic texture (elongate crystals enclosed by another mineral). It is often found in dolerites and gabbros. The term subophitic is sometimes used to descibe an ophitic texture where chadacrysts are not entirely enclosed by the oikocryst. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

An igneous rock texture in which large crystals are set in a finer-grained or glassy groundmass. Porphyritic textures occur in coarse, medium and fine-grained igneous rocks. Usually the larger crystals, known as phenocrysts, formed earlier in the crystallisation sequence of the magma. A porphyritic texture in which there is a continuous variation in crystal size is termed seriate. Hypabyssal rocks with porphyritic textures are sometimes termed porphyrys. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A porphyroblast is a crystal grown during metamorphism that is significantly larger than the surrounding matrix. Porphyroblasts are commonly euhedral crystals and have often deflected the surrounding foliation. They also are frequently associated with strain shadows. Porphyroblasts form from mineral phases in a metamorphic rock that have more limited nucleation than the matrix minerals. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A porphyry is an intrusive igneous rock with a porphyritic texture, having larger crystals embedded within a finer-grained groundmass. The term is a textural rock classification which is not part of the IUGS classification system. It is, however, still in common usage, and is usually only applied to hypabyssal acid or intermediate rocks. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems. Pyroxenes have the general formula XY(Si,Al)2O6 (where X represents calcium, sodium, iron+2 and magnesium and more rarely zinc, manganese and lithium and Y represents ions of smaller size, such as chromium, aluminium, iron+3, magnesium, manganese, scandium, titanium, vanadium and even iron+2).
[Source: en.wikipedia.org/]

A quartzite is a granular metamorphic rock dominated by quartz (>90% by volume). Accessory minerals include feldspar and detrital heavy minerals such as titanite and rutile. Some quartzites exhibit a foliation due to grain flattening fabrics. Quarzite is a metamorphosed quartz arenite (sandstone) and strictly speaking should not be used for rocks containing higher proportions of feldspar formed by the metamorphism of feldspathic arenite and arkose. The term psammite is reserved for such rocks, however, often the prefix meta- is used in combination with the protolith name, for example metasandtone, meta-arkose. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A reaction rim is a mantle of one mineral completely or partially surrounding a different existing mineral that demonstrably originates due to reaction and replacement of the original phase. Reaction rims indicate that mineral with the rim was out of equilibrium with its surroundings. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

S

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than gravel and coarser than silt, per the ISO system this is between .063 and 2 mm. [Source: en.wikipedia.org/ ]

A sandstone, also known as an arenite or arenaceous rock, is a clastic sedimentary rock comprised mainly of sand grade particles between 0.0625 and 2 mm. Sand particles can be further subdivided into very fine (0.0625 to 0.125 mm), fine (0.125 to 0.25 mm), medium (0.25 to 0.5 mm), coarse (0.5 to 1.0 mm) and very coarse (1.0 to 2.0 mm). Sandstones are subdivided on the basis of their grain types. Grains can be quartz, feldspar or rock fragments (lithic). Sandstone types include quartz arenite, subarenite, litharenite, arkose and subarkose. Litharenites can be further subdivided according to the type of lithic clast into sedarenite, volcanic arenite and phyllarenite. Sandstones containing more than 15% fine-grained matrix are known as greywackes. The term orthoquartzite can be used for quartz arenites that are well sorted and have well rounded grains, whilst the term protoquartzite for quartz-rich sandstones with less rounded sand grains and finite fine-grained matrices (<15 vol%). Protoquartzites are equivalent to quartz subgreywackes. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Seriate is textural term applied to crystalline rocks that describes crystal sizes that vary continuous from the smallest to largest. Seriate is an inequigranular texture. Seriate is also used to describe an irregular crystal boundary between crystals within a metamorphic rock arising due to grain boundary migration during grain coarsening. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

The serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate ((Mg, Fe)3Si2O5(OH)4) minerals; they may contain minor amounts of other elements including chromium, manganese, cobalt or nickel. In mineralogy and gemology, serpentine may refer to any of 20 varieties belonging to the serpentine group. Owing to admixture, these varieties are not always easy to individualize, and distinctions are not usually made. There are three important mineral polymorphs of serpentine: antigorite, chrysotile and lizardite. [Source: en.wikipedia.org/ ]

A siltstone is a clastic sedimentary rock dominated by silt grade detrital grains between 1/16 and 1/256 mm in size. Siltstones can also contain minor clay-grade (argillaceous) materials. Siltstones are deposited under relatively quiescent conditions and are often distal to sources of the detrital grains. Interlaminated siltstones and mudstones/shales are often represent distal turbidites or lacustrinal deposits. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A skarn is a term to describe a rock formed by metasomatic replacement of a protolith and is most commonly used for calc-silicate rocks formed by the replacement carbonates by contact or regional metamorphism. A wide range of metal ore deposits are associated with skarns including W, Sn, Mo, Cu, Fe, P-Zn and Au ores. Generally Fe and Au skarns are associated with basic to intermediate magmas, whilst Cu, Pb, Zn, W, Sn and Mo are associated with calc-silicate intrusions. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Skeletal is a term used to describe the habit of euhedral to subhedral crystals in igneous rocks containing crystallographically orientated hollows and gaps. Skeletal crystals can be described as re-entrant. Typically the voids within skeletal crystals are filled with groundmass materials. Completely enclosed voids are effectively melt inclusions. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A spherulite is a cluster of radiating acicular or lath-like crystals found in igneous rocks. Spherulites form by crystallisation under large supercooling where nucleation is impeded. To grow crystals use submicroscopic pre-existing nuclei to begin their growth. Spherulites are most common in devitrified rhyolites known as pitchstones where feldspar crystals from radiating clusters, or even botryoidal-like bands of radiating acicular crystals. Often spherulites in older rhyolites have recrystallised to form granular masses and only their outlines are visible. Glassy black rocks with abundant white spherulites have been coined "snowflake obsidian". Rocks with abundant spherulites are often prefixed with "spherulitic". [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Spinifex is a textural term applied to elongate olivine dendrites within ultramfic rocks such as komatiites. Spinifex olivines indicate growth under large supercooling and significant disequilibria. Within komatiites spinifex olivines grow since the high initial temperature of these Mg-rich melts destroys the crystallisation nuclei required for initial growth of olivine impeding crystallisation until lower temperature. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Syenite is a coarse-grained alkaline igneous rock dominated by essential alkali feldspar. Mafic minerals are usually present in small amounts and include clinopyroxenes such as augite, diopside, aegirine-augite and aergirine, and hornblende and biotite. Plagioclase can also be present. Titanite is a common accessory mineral. Syenite can contain both feldspathoids, including nepheline, sodalite and/or leucite, olivine or quartz. Feldspathoid syenites often have alkali pyroxenes, such as aegirine-augite and aegirine, or alkali amphiboles, such as reibeckite. Larvikite can be a variety of syenite dominated by perthitic feldspars with a moonstone schiller. Shonkinite is a mafic-rich potassic variety of syenite. Trachyte and phonolite are the fine-grained equivalents of syenite. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

T

Tephrite is a fine-grained alkaline igneous rock dominated by essential plagioclase, and feldspathoid, usually nepheline, leucite and/or analcite. Tephrites also contain a mafic mineral, usually a clinopyroxene such as augite, aegirine-augite, or aegirine. Pyroxene and feldspathoids usually occur as phenocrysts. Phonolitic tephrites also contain alkali feldspars. Basanite is an olivine-bearing tephrite. The coarse-grained equivalent to a tephrite is a feldspathoid gabbro or monzogabbro, also termed a theralite. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Tholeiites are a chemical sub-type of basalt defined on their silica content. Basalts that are silica saturated are known as olivine tholeiites, those that are silica oversaturated are termed quartz tholeiites. Tholeiites lack feldspathoids. Silica undersaturated basalts are termed alkali basalts. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/ ]

A trachytic texture consists of a subparallel arrangement of lath-shaped feldspars in the groundmass of an igneous rocks. Trachytic textures are common amongst trachytes but are observed in other rock types, in particular the trachybasalts and phonolites. Trachytic textures are often attributed to flow orientation, however, there is little evidence to support this. Macroscopic trachytic textures visible with the naked eye are sometimes called trachytoid textures. Trachytic textures can be subdivided into pliotaxitic, if interstitial regions are crystalline, or hyalopilitic, if they are occupied by glass. These terms are, however, rarely encountered. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/ ]

Travertine is a form of limestone formed by chemical precipitation of aragonite and/or calcite from aqueous solution. The most abundant accumulations of travertine occur around hot springs and soda-lakes. Travertine is often banded and/or fibrous and often contains detrital and sometimes biogenic clasts. Precipiation of carbonates on clasts often leads to concentric structures. Stomatilitic structures can be present due to precipitation by micro-organisms. Travertine also forms precipitated carbonates within caves and makes up speleothems such as stalactites and stalagmites, and the matrix of many cave breccias. The term tufa is used to describe highly porous travertine. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A tuff is a lithified volcanic ash and is produced by explosive volcanic eruptions. Tuffs are dominated by ash-sized (<2 mm) grains with <25 vol% lapilli (2-64 mm) or blocks or bombs (>64 mm) clasts. Most tuffs form due to the accumulation of ash distal to the volcanic vent by direct airfall from the volcanic plume. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

V

A variolitic texture consists of a fan-like arrangement of elongate, sometimes branching, crystals. Variolites are often arranged in several layers with one set of variolites growing on top of the previous generation. Variolitic textures form by heterogeneous nucleation under larger supercooling and are this similar to spherulites. They are most commonly encountered in dolerites. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

A vesicle is subspherical or elongate cavity within an igneous rock formed by expansion of gas. Vesicles occur in volatile-bearing magmas at low pressures. Gas exsolves from the magma once its partial pressure exceeds the confining pressure. Vesicles are common in volcanic rocks and a fundamental part of pyroclasts such as scoria and pumice. Rocks containing abundant vesicles are termed vesicular. [Source: wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/ ]

Z

A texture developed in solid-solution minerals and characterized optically by changes in the colour or extinction angle of the mineral from the core to the rim. This optical zoning is a reflection of chemical zoning in the mineral. For example, a plagioclase can be zoned from a Ca-rich core to an Na-rich rim. Zoning results from the mineral's inability to maintain chemical equilibrium with a magma during rapid cooling; the zonation represents a frozen picture of the continuous reaction series for that mineral. [Source: www.oxfordreference.com/ ]